Malcolm Turnbull has defended his candidate in the crucial Longman by-election, who wrongly claimed he'd been awarded an Australian Service Medal for his time in the air force.

The prime minister said Trevor Ruthenberg had made an "innocent mistake for which he's very embarrassed and he's apologised profusely".

Mr Ruthenberg's biography on the Queensland parliamentary website claimed he'd been given the medal, but he had, according to The Courier-Mail, received the Australian Defence Medal, given to all military personnel who serve for four years.

Mr Ruthenberg, considered a good chance of winning the South East Queensland seat, said the mistake wasn't intentional, and he has no idea how it happened.

Trevor 'big Trev' Ruthenberg. Picture: AAP

He told a media conference it was a "simple mistake" that occurred some time ago and he took immediate steps to correct it when it was discovered.

"I apologise unreservedly," he said.

"I'm owning the mistake. Part of being a person of integrity is to admit to a mistake."

He said he'd apologised to Mr Turnbull.

And, he had never worn the Australian Service Medal, incorrectly mentioned on the website.

Malcolm Turnbull is seen making an address at the Queensland Media Club. Picture: AAP

Mr Turnbull spoke to the man they call "big Trev" this morning and was satisfied it was an honest mistake.

"He mis-described the medal that he had been awarded," Mr Turnbull told Melbourne radio.

"Big Trev is as honest and straight as he is big. And he is big.

"I've spoken to him about it and am satisfied it is an honest mistake."